High thermal conductivity aluminum nitride (AlN) is suitable for many applications, such as packaging components for electronic circuitry. However, carbothermally produced aluminum nitride often contains impurities such as silicon, calcium, chromium, and iron, which are believed to cause color variability and lower thermal conductivity of the sintered aluminum nitride.
Kuramoto, et al., Ygoyo Kyokai Shi, 93(9): 517-22 (1985), disclose low metals impurities for achieving high thermal condutivities in aluminum nitride. U.S. Pat. No 4,618,592, issued to Kuramoto, et al., also disclose concentrations of metals for high thermal conductivity aluminum nitride. Low metals contamination in aluminum nitride is also important for obtaining a consistent color in final aluminum nitride sintered parts. Skeele, et al., "Evaluation of Properties/Performance Relationships for Aluminum Nitride," Proc. 3rd Int'l Conf. Ceramic Powder Processing Sci. (1990), disclose the effect of trace metals on the color of sintered aluminum nitride parts. It is generally accepted that contamination by metals causes color variation and limits thermal conductivity in aluminum nitride.
Typically, methods of carbothermally producing aluminum nitride either do not provide for removal of these impurities or require additional processing steps which can significantly diminish the yield of the carbothermal reaction. In one method of producing aluminum nitride, disclosed in U.S. Pat. No. 2,962,359, carbothermal reaction to form aluminum nitride is performed at a temperature in a range of between about 1650.degree. C. and about 1750.degree. C. to diminish formation of AlCN. Reaction between aluminum oxide and carbon is thereby more complete. However, only carbon, aluminum oxide and AlCN are reduced by this process. The presence of trace impurities, such as silicon, calcium, chromium and iron, are not diminished by conversion of aluminum oxide to aluminum nitride followed by oxidation for the removal of excess carbon.
Another method of producing aluminum nitride by carbothermal reaction, disclosed in U.S. Pat. No. 3,032,398, includes pelletizing finely comminuted aluminum oxide and carbon with a binder comprising an aluminate of lime such that the pellets are sufficiently porous to permit nitrogen to pass to the center of the pellets. The binder is removed during carbothermal reaction by conducting the pellets through vertical retorts of a reactor, at the top of which is disposed an expansion chamber for removing gross amounts of volatilized impurities, such as calcium, from the binder. However, trace impurities which are not transported away from the pellets during carbothermal reaction remain.
Therefore, a need exists for a new method of producing aluminum nitride by carbothermal reaction which overcomes or minimizes the above-listed problems.